Why
color management?

Obtaining
predictable color reproduction in the digital darkroom can be
a challenge because each device-- digital camera, scanner, monitor, or
printer-- responds to or produces color differently. If you limit your
operations to your own well-characterized equipment and follow the
procedures
outlined in Monitor
calibration and
Printer
calibration, you can get reasonably good results without
color
management.
(The operating system performs a certain amount covertly, in the
background.)
But you may want to go further-- to push the envelope. You may want to

improve the color match between your monitor and
printer.

work with fine art papers or nonstandard inks, for example,
archival pigment-based
inks from independent manufacturers.

send out an image to be printed, for example, on a large
format LightJet
printer.

reproduce color as precisely as possible-- for example, for
fashion, weddings,
or art reproduction.

To meet these goals you'll need to get involved with the set
of software
packages and procedures known as color management. There's a learning
curve,
but it will ultimately simplify your workflow.

Inside
color management
The latest on what color management is, how it works, and what tools
are
available. David Broudy, Electronic Publishing, June, 2001. A nice
thoughtful
article on the RIT website.

PhotoShopNews.com
Excellent source of news and information on color management and
digital techniques; not for Photoshop users only.

The
ColorShop Color Primer: an introduction to the
history of color, color
theory, and color measurement by Fred Bunting, Light Source Computer
Images,
Inc., an X-Rite Company. A 116 page PDF document-- a complete textbook
in itself.

Links
from the Colour
Imaging Group, School of Printing and Publishing, London College of
Printing.
A very nice set of references.

Real
World Color Management: Industrial-strength
production techniques,
by Bruce Fraser, Fred Bunting, and Chris Murphy. Paperback, 560 pages.
The closest thing to a Color Management Bible. Buy it if you want to go
into real depth. But as its subtitle "Industrial-strength production
techniques"
indicates, much of the material covers prepress applications (CMYK,
etc.),
which are of only indirect interest to photographers.

Mastering
Digital Printing: The Photographer's and Artist's
Guide to High-Quality
Digital Output, by Harald Johnson. Paperback, 400 pages. Perhaps more
for
artists than photographers, it has a good introduction to color
management.

Color
Science: Concepts and Methods, Quantitative Data
and Formulae,
by Günther Wyszecki and W. S. Stiles. Paperback, 968 pages. A
collection
of scientific review papers-- not for the casual reader. This is the
reference
I use to verify questionable statements on color and color management
in
popular literature. It's the source.

The
eye's response to color

The
retina of the human eye has two categories of light receptor: rods,
which are active in dim light and have no color sensitivity, and cones,
which are active in bright light and provide us with our ability to
discriminate
color. You probably learned that the three types of cone are sensitive
to red, green, and blue (R, G, and B). Close,
but no cigar. The relative sensitivity
of the three receptors
for the "normal" human eye, designated by Greek letters beta, gamma and
rho (β, γ,
and ρ), is illustrated by the blue, green,
and red curves on the right.
Although
the beta and gamma sensors correspond closely to blue and green, the
rho
sensor (the red curve) isn't even close to red. An ink with the same
reflectivity
spectrum would appear yellow-orange.

The eye/brain discriminates color by processing the relative
stimuli in the three sensors. R, G, and B are used as additive primary
colors because their distribution across the visible spectrum produces
a wide-gamut color image, not
because they
match the eye's response. Fewer than three colors is insufficient.
Additional
colors offers some advantage-- that's why recent inkjet photo printers
have 6 to 8 colors. Combining three colors-- even monochromatic
(spectrally
pure) colors produced by lasers-- can produce most, though
not all,
of the colors the eye can see.

How is this known? A set of experiments is run using
a split screen.
Half is illuminated by a monochromatic light source with variable
wavelength.
The other half is illuminated by an adjustable combination of red,
green
and blue, which can be produced by lasers or by filters, which have a
broader
spectrum than lasers. If the two halves of the screen can be matched
with
some combination of the R, G, and B lights, then the color of the pure
monochromatic light is within the gamut
of colors defined by the three light sources. If no match is possible--
if white light must be added to the monochromatic source to provide a
match--
then the color of the monochromatic source is out
of gamut. This experiment shows that no
combination
of three real light sources can duplicate the full gamut of human
vision.

Monitors with three phosphor colors (RGB) have
limited color gamuts;
printers with four ink colors (CMYK = cyan, magenta, yellow, black)
have
even smaller gamuts. Printers with additional colors-- 6 to 8 are not
uncommon--
have larger gamuts. The eye's peculiar response has consequences for
the
discipline called color
science,
which has arisen to quantify human vision. A detailed exposition would
overwhelming, but a few aspects, which are widely discussed but poorly
understood, are important for color management.
.

...An
excellent opportunity to
collect high quality photographic prints and support this website

.

Color
science in a nutshell

In order to quantify human
color vision, the CIE
(Commission
Internationale de L'Éclairage)
has established a set of imaginary "red," "blue," and "green" primary
colors
that, when combined, cover the full gamut of human color vision, i.e.,
a combination of the three can match any monochromatic light source.
These
color primaries (shown on the left) have a curious property-- they have
negative energy in portions of their spectra, i.e., they are not
physically
realizable.

The combinations of these three "light" sources required to match
monochromatic
(spectral) light are determined experimentally (by performing
mathematical
matrix transformations of the results of the split screen matching
experiment).
The curves for these combinations, shown on the right, are called the Color
Matching Functions for the Standard Colorimetric
Observer.
They are designated x¯,
y¯,
and z¯,
and they never have negative values.Example:
450 nm monochromatic light can be matched with x¯ = 0.34,
y¯
= 0.04, z¯ = 1.77. The
Color Matching Functions
are used to derive the XYZ
Tristimulus Values
that uniquely define an object's colorimetry;
two objects with the same tristimulus values have identical color
appearance
when viewed under the same conditions. The X, Y, and Z tristimulus
values
are calculated by integrating (summing) the product of the spectral
reflectivity,
the illuminant (the light source), and the corresponding color matching
function from 380 to 700 nanometers (nm). Although the tristimulus
values
uniquely define an object's color, they do not define the eye's
response
to the color, which depends on the environment and the eye's
adaptation.

If two objects with different spectral reflectivities have the
same
color appearance (tristimulus values) under one light source, they are
said to be metamerically
matched.
If they exhibit a marked difference under another light source (as with
two pieces of cloth that look identical in a shop but different
outdoors),
they suffer from viewing
illuminant sensitivity,
frequently called metamerism.
(This definition is misleading, but it has become so prevalent I won't
try to fight it.) This phenomenon is most visible with neutral colors
(grays).
It is a problem with many color printers, particularly with Black
&
White prints; it was particularly severe with the Epson 2000P, but
newer printers (2005 and later) have largely solved the problem. In
actuality,
metamerism, properly defined, is not a bad thing: it enables a
combination
of just a few inks to take on the colors of a wide range of objects.

.XYZ tristimulus values are important because they form the
basis
of the familiar but often misunderstood CIE 1931 chromaticity
diagram,
shown on the right. This diagram is based on normalized
tristimulus
values, x, y, and z, where, x = X/(X+Y+Z), y = Y/(X+Y+Z), and z =
Z/(X+Y+Z).
This normalization (division by X+Y+Z) removes the brightness from the
diagram so that only two coordinates, x and y, are needed to define
chromaticity.
z = 1-x-y would be redundant. Since Y is closely related to perceived
luminance,
colors are sometimes expressed as xyY tristimulus values.

The colors in the
diagram are not accurate, but the representation produced by Gamutvision is
about as good as
can be obtained with the limited color gamut of a computer monitor.

The horseshoe line starting at 400 nm on the lower
left and wrapping
around the top to 700 nm on the right is called the spectrum
locus. It represents the pure spectral
colors-- the beautiful,
intense colors produced by a prism in clear sunlight. The screen image
is but a pale approximation. The straight line
connecting the endpoints of the horseshoe is called the purple
boundary. The full gamut of human
vision lies within this
figure. The vertical axis gives an approximate indication of the
proportion
of green; the horizontal axis moves from blue on the left to red on the
right. The location of white depends on the illuminant color
temperature.
Some typical values (from efg):

Incandescent
lamp

2856o
K

x,y
= 0.448, 0.407

Direct
sunlight

5335o
K

x,y
= 0.336, 0.350

Overcast
sky, D65

6500o
K

x,y
= 0.313, 0.329

The 1931
chromaticity diagram is not without its
flaws. The distance between
just noticeable color differences (called ΔE) is
much greater in the
green region on the top than at the bottom; it is not perceptually
uniform.
For this reason, CIE has defined additional color spaces, particularly
CIELUV
(1976) and CIELAB
(1976), which
represent colors more uniformly. But the 1931 diagram persists because
of historical inertia. All the CIE color spaces encompass the full
gamut
of human vision and all are device-independent.
Bruce
Lindbloom presents the
equations
for converting between them. But unlike RGB color spaces, they aren't
intuitive.
The precise meaning of their coordinates is difficult to visualize and
they contain values outside the gamut of human vision. Hence they
aren't
used as working color spaces for image editing. They play a vital but
invisible
roll in color management; you don't need to understand their details to
manage color effectively.

sRGB vs.
Adobe RGB (1998) color gamutsshown in L*a*b* color
(shows
that there ismore than meets the eye in the CIE 1931 xy
diagram)Image produced by Gamutvision.
.

.The CIE 1931 chromaticity diagram is used to illustrate the gamut
of a device or a color
space--
the
range of reproducible colors generated by a specific set of primaries.
(Color space profiless also include a specification for gamma and white
point.) For three-color additive devices, gamut is defined by the
triangle
formed by the x, y chromaticity values of the Red, Green and Blue (RGB)
primary colors. Colors inside the triangle can be reproduced; colors
outside
can't; they are out
of gamut.
The above figure on the right illustrates the gamut for the sRGB
color
space. sRGB was designed to match the gamut ot a typical monitor with
gamma
of 2.2 and white point set to 6500K (D65). It is the
de facto standard
for the Internet and the Windows operating system.

The gamut of sRGB, shown on the right inside the gamut of
Adobe RGB (1998), is quite limited. It maintains a distance
from the line of purples and is weak in green and cyan, although this
weakness
is greatly exaggerated by the distortion of the 1931 diagram. In
monitors,
the gamut is limited by the phosphors, which are chosen for brightness,
longevity, low cost, and low toxicity. Ideal phosphors-- with colors
located
near 450, 520 and 650 nm on the spectrum locus-- don't exist. Most
monitors
with P22 phosphors have similar gamuts.

Printers, whose colors are based on variants of CMYK
(cyan, magenta,
yellow, black) subtractive primaries, have gamuts whose shape is more
complex
than a simple triangle-- often somewhat hexagonal with additional
vortices
at the Cyan, Magenta, and Yellow primaries. Because inks have imperfect
spectra
and are somewhat opaque, dark or light colors tend to have smaller
gamuts
than middle tones. Most four-color CMYK printers have smaller gamuts
than
monitors, but high quality inkjet printers with more than four colors
(typically
with the addition of light C and light M) may have larger gamuts. Color
slide
films have considerably larger gamuts.

Now we approach one of the key issues in color
management: How is color
represented in computers? We focus on RGB color spaces because they are
used for digital image files, and there are a good many of them. Other
color models (CMYK, HSV and HSL) are discussed elsewhere
on this site and in Microsoft's
MSDN library.

In 24-bit RGB color spaces, color is described by
three 8-bit bytes,
each of which can take on values 0 through 255. Pure red is (255,0,0),
green is (0,255,0), blue is (0,0,255), black is (0,0,0), and white is
(255,255,255).
Some programs like Picture
Window Pro and
Photoshop CS can utilize 48-bit color that allows values of 0 through
65,536
for each primary color. 48-bit color eliminates banding that can occur
when colors are adjusted in the 24-bit mode.

The
numeric RGB values of an image file have no clear, unambiguous meaningunless
they are associated with a color space.

But what exactly is meant by
"pure" R, G and B? For
a given color space, "pure" R, G and B are the primary colors, located
at the apexes of the gamut triangle: R on the right, G on the top, and
B on the left. To accommodate the wide
range of gamuts in
different devices-- digital cameras, film, scanners, monitors, and
printers,
a variety of color spaces has been developed. The de facto standard for
the Internet and Windows, sRGB, has a limited gamut corresponding to a
typical CRT monitor. Other color spaces have larger gamuts--
corresponding
roughly to high quality printers or to film. Some have extremely large
gamuts, covering most of the colors the eye can see and some it can't.
Maintaining consistent color appearance in the translation between
different
devices and color spaces is no easy task; color management provides a
reasonably
sane and practical solution. But it's is no panacea. Even the most
sophisticated
system can't make two devices with different gamuts display exactly
the same colors; it can't make a monitor or CMYK printer display
Velvia-saturated
colors.

In a color-managed workflow, the color response of each
device, each
image file, and each image in the computer's active memory is
characterized
by a file called an
ICC profile.
ICC profiles have the extension ".icm" and are stored in specific
locations
on Windows computers.

Windows
98, ME

Windows\System\color

Windows 2000

WinNT\System32\Spool\Drivers\Color

Windows XP

WINDOWS\system32\spool\drivers\color

You
may need to do some
bookkeeping in these folders because problems can
arise (especially with pre-XP versions of Windows) when more than about
30 profiles are present. Since profiles are loaded by profile creation
programs such as MonacoEZcolor, Windows itself, image editors, and
device
drivers, they can easily proliferate. I recommend creating a folder
called
"Unused profiles" for profiles you don't use. If nothing else, it will
shorten the drop-down lists. You can obtain a printout of profile
descriptions
from Picture Window Pro by clicking on File,
Color
Management... to bring up the Color
Management dialog box,
then clicking Print Profile
List... ICC profiles can also be embedded as tags
within image files:
TIFF, JPEG, PNG,
and BMP are supported by most ICC-aware image editors.

ICC
Profiles
consist primarily of tables that relate numeric data, for example, RGB
(222,34,12), to colors expressed in a device-independent CIE color
space
called a Profile
Connection Space (PCS)--
either CIE-XYZ or CIELAB. The colors may be the objects
sensed by
a scanner or produced by a printer or monitor. They can also refer to
one
of the numerous color
spaces.
Monitor profiles have the same format as color space profiles. Profiles
may contain additional data, such as a preferred rendering intent and gamma,
Monitor profiles often contain instructions for loading video card
lookup
tables, i.e., for calibrating the monitor.

The heart of color management is the translation or gamut
mapping between devices with different color gamuts
and files with
different color spaces. Mapping functions are shown in the yellow boxes
in the illustration, above. They are performed by a
Color
Matching Module or Method
(CMM),
also called a Color
Engine, using data in the profiles.
The CMM combines the
input and output profiles, both of which are referenced to a PCS, to
perform
a direct conversion between the devices or color spaces. It interpolates
data in printer profile tables, which would be prohibitively large if
all
possible color values were included.

Picture Window Pro uses either the Windows default
color engine, ICM
2.0, or an alternative engine, LittleCMS. Adobe
Photoshop has its own color engine, ACE. Color
engine mappings
may be called from ICM-aware programs or device drivers. You must be
aware
of where the translation takes place in your environment. If you are
careless,
mapping can take place twice (or not at all), with undesirable results.
We will show examples.

For reference, ColorSync
is Apple's color engine. ColorMatch
RGB is Apple's default color space, with gamma =
1.8 and gamut
between sRGB and Adobe RGB (1998). Apple
RGB
is an older color space with a narrower gamut. Easy to get confused.

Gamut
mapping is performed with
one of the four rendering
intents (gamut mapping algorithms) recognized by
the ICC standard
and by Windows ICM 2.0. The rendering intent determines how colors are
handled that are present in the source but out of gamut in the
destination.
Since there are several nomenclatures for gamut mapping, I use a color
code to distinguish the sources: ICC,
Windows
ICM 2.0, Picture
Window Pro.
I'll generally stick with the ICC
nomenclature.

Perceptual,
also called Picture
or Maintain Full Gamut.
This
is PW Pro's default, and is generally recommended for photographic
images.
The color gamut is expanded or compressed when moving between color
spaces
to maintain consistent overall appearance. Low saturation colors are
changed
very little. More saturated colors within the gamuts of both spaces may
be altered to differentiate them from saturated colors outside the
smaller
gamut space. In the diagram on the right, the left and right of the
color
space blocks represents saturated colors; the middle represents neutral
gray. Perceptual rendering applies the same
gamut
compression to all images, even when the image contains no significant
out-of-gamut colors. Bruce
Fraser points out that for an image with unsaturated colors,
e.g.,
with pastels, Relative Colorimetric rendering may produce a slightly
more
accurate result. Perceptual gamut mapping is mostly
reversible;
it is most accurate in 48-bit color. None of the other rendering
intents
is reversible.

.

Relative
Colorimetric, also called
Proof
or Preserve Identical
Color and White Point.
Reproduces in-gamut colors exactly and clips out-of-gamut colors to the
nearest reproducible hue. Not reversible. See diagram.
Bruce
Fraser says, "Look
at the relative gamuts of your source and destination: The same image
may
need different rendering intents for different output process. For
example,
an image might benefit from perceptual rendering when printed to an
inkjet
printer, but when the same image is going out to the much larger gamut
of a film recorder, relative colorimetric rendering might work much
better.
If an image doesn't contain any important strongly saturated colors,
you'll
probably get a better result using relative colorimetric rendering than
you would using perceptual."

.

Absolute
Colorimetric, also called
Match
or Preserve Identical
Colors.
Reproduces in-gamut colors exactly and clips out-of-gamut colors to the
nearest reproducible hue, sacrificing saturation and possibly
lightness.
On tinted papers, whites may be darkened to keep the hue identical to
the
original. For example, cyan may be added to the white of a
cream-colored
paper, effectively darkening the image. Rarely of interest to
photographers.

Saturation,
also called Graphic
or Preserve Saturation.
Maps
the saturated primary colors in the source to saturated primary colors
in the destination, neglecting differences in hue, saturation, or
lightness.
For block graphics; rarely of interest to photographers.

Rendering intents don't always perform according to the
textbook description. You can view their actual performance with Gamutvision—
a powerful utility written by the author of this tutorial.

ICC
profiles reference the CIE device-independent color
spaces-- CIEXYZ
and CIELAB. When gamut mapping is performed, tables from the source and
destination profiles are combined; gamuts are mapped directly. This
minimizes
loss of image quality. More on rendering intents can be found in
articles
from Microsoft,
ProfileCity.com
and Bruce
Fraser.

Gamut mapping
links (for geeks) Gamut mapping is a complex
topic, particularly for Perceptual rendering intent. The details of the
are contained within the profile. There are a great ways of performing
Perceptual rendering. Here are some highly technical papers, guaranteed
to generate more questions than they answer. Like what techniques are
actually
used in your profiles? This doesn't sound anything
like the familiar
"you push the button, we do the rest" marketing hype.

Gamut
mapping: The transformation
that takes place when an image is transferred between formats or
devices,
for example,

from one color space to another.

from an image in memory to a monitor.

from an image in memory to a printer.

A color-managed
workflow uses
ICC-aware programs to do two things: (1) recognize color spaces and
device
profiles, and (2) apply the appropriate gamut mapping when transferring
images. That's all! But with color management, "the devil is in the
details,"
and what details! They include

How to build or obtain profiles.

How to evaluate their quality.

How to set up workflows to utilize them properly.

What working color space to choose.

How to deal with images that don't have embedded profiles.

What rendering intent(s) to choose for "reasonable" color
rendition.

Most importantly, how to get the best subjective match
between the monitor
image and the print.

Images
and text copyright (C) 2000-2013 by Norman Koren. Norman Koren lives
in Boulder, Colorado, where he worked in developing magnetic recording
technology for high capacity data storage systems until 2001. Since 2003 most of his time has been devoted to the development of Imatest. He has been involved with photography since 1964.